The Romans killed the men who were taken

Here are two clauses: 

  a. The main clause, The Romans killed the men

  b. The subordinate clause, who were taken

The word who is a pronoun, for it takes the place of the noun men.  It also connects the subordinate clause who were taken with the noun men.  Hence the clause is an adjective clause.  A pronoun that connects an adjective clause with a substantive is called a relative pronoun, and the substantive for which the relative pronoun stands is called its antecedent.  The relative pronouns in English are who, whose, whom, which, what, that.

221. The relative pronoun in Latin is qui:\, quae\, quod\, and it is declined as follows: 

SINGULAR                  PLURAL
MASC.    FEM.     NEUT.      MASC.      FEM.       NEUT. 
Nom.  qui:     quae    quod      qui:       quae      quae
Gen.  cuius   cuius   cuius     quo:rum   qua:rum   quo:rum
Dat.  cui     cui     cui       quibus    quibus    quibus
Acc.  quem    quam    quod      quo:s     qua:s     quae
Abl.  quo:     qua:     quo:       quibus    quibus    quibus

1.  Review the declension of is\, Sec. 114, and note the similarity in
the endings.  The forms qui:\, quae\, and quibus\ are the only forms
showing new endings.

NOTE.  The genitive cuius\ and the dative cui\ are pronounced c[oo]i’y[oo]s (two syllables) and c[oo]i (one syllable).

222. The Relative Pronoun is translated as follows:[1]

         MASC.  AND FEM.  NEUT. 
  Nom. who, that which, what, that
  Gen. of whom, whose of which, of what, whose
  Dat. to or for whom to or for which, to or for what
  Acc. whom, that which, what, that
  Abl. from, etc., whom from, etc., which or what

[Footnote 1:  This table of meanings need not be memorized.  It is
inserted for reference when translating.]

a. We see from the table above that qui:\, when it refers to a person, is translated by some form of _who_ or by _that_; and that when it refers to anything else it is translated by _which, what_, or _that_.

_223._ Note the following sentences: 

The Romans killed the men who were taken The Romans killed the woman who was taken Romani interfecerunt viros qui capti sunt\ Romani interfecerunt feminam quae capta est\

In the first sentence who (qui) refers to the antecedent men (viros), and is masculine plural.  In the second, who (quae) refers to woman (feminam), and feminine singular.  From this we learn that the relative must agree with its antecedent in gender and number.  In neither of the sentences are the antecedents and relatives in the same case. Viros\ and feminam\ are accusatives, and qui\ and quae\ are nominatives, being the subjects of the subordinate clauses.  Hence